Strategies for handling deco schedule deviations

[Rick Murchison]

In a recent thread on acceptable deviations from stop depths, I got interested in exploring and discussing strategies for handling deviations of all sorts from the planned deco schedule.
In the real world deviations from the planned schedule occur on every dive. Usually those deviations fall within what we'd consider an acceptable range and no change to the schedule is required. But sometimes the deviation(s) cross a threshold requiring a mod to the schedule.
What I'd like to explore here is a two part question:
What are some examples of thresholds requiring a change to the schedule?
and
What is your strategy for handling the change?
--
Note: I'm not interested in inter- or intra-agency squabbles over "proper" procedures, but rather your personal reasoning for your strategy(ies).
--
OK... first example: Blown deep stop - Profile was a free boat, 215' dive, 15 min BT on 18/45.
Schedule was:
Dec to 200ft (4) Trimix 18/45 50ft/min descent.
Dec to 215ft (4) Trimix 18/45 60ft/min descent.
Level 215ft 10:45 (15) Trimix 18/45 1.35 ppO2, 83ft ead, 103ft end
Asc to 130ft (17) Trimix 18/45 -30ft/min ascent.
Stop at 130ft 0:10 (18) Trimix 18/45 0.89 ppO2, 43ft ead, 57ft end
Stop at 120ft 1:00 (19) Trimix 18/45 0.83 ppO2, 39ft ead, 51ft end
Stop at 110ft 1:00 (20) Trimix 18/45 0.78 ppO2, 34ft ead, 46ft end
Stop at 100ft 1:00 (21) Trimix 18/45 0.72 ppO2, 29ft ead, 40ft end
Stop at 90ft 1:00 (22) Trimix 18/45 0.67 ppO2, 25ft ead, 35ft end
Stop at 80ft 2:00 (24) Trimix 18/45 0.62 ppO2, 20ft ead, 29ft end
Stop at 70ft 2:00 (26) Trimix 18/45 0.56 ppO2, 15ft ead, 24ft end
Stop at 60ft 3:00 (29) Trimix 18/45 0.51 ppO2, 11ft ead, 18ft end
Stop at 50ft 4:00 (33) Trimix 18/45 0.45 ppO2, 6ft ead, 13ft end
Stop at 40ft 6:00 (39) Trimix 18/45 0.40 ppO2, 1ft ead, 7ft end
Stop at 30ft 8:00 (47) Trimix 18/45 0.34 ppO2, 0ft ead, 2ft end
Stop at 20ft 6:00 (53) Oxygen 1.60 ppO2, 0ft ead
Stop at 10ft 10:00 (63) Oxygen 1.30 ppO2, 0ft ead
Surface (63) Oxygen -30ft/min ascent.

Never mind the details, (it involved a screwup by both my buddy and me and a $money$ decision at depth) at 15 min run time I made a rapid (about 60fpm) ascent from 215 to 100’ – so here I am at 100’ at 17 minutes vice 20 minutes run time, but with the full 15 min BT load.
At this point I do a little Haldanean/Navy table thinking that goes like this… “If I were diving the Navy Air tables I’d be making a 60fpm ascent all the way to 30’… at 18% oxygen I don’t have that much more inert gas in the mix, so I shouldn’t be in too big a mess…”
Now what? Any blown stops require something, eh?
I decide that I’ll stay at 100, pick up the deep portion of the schedule at 21 minutes run time, but play a little Navy Table on the other end and add 10 minutes shallow – 5 at 20 and 5 more at 10.
Worked J and I’m happy with that strategy for a couple of missed deep stops.
One more point – and grist for another thread – make those money decisions topside, before you ever get in the water.
Rick

 

[karstdvr]

Interesting problem. It would be cheating for someone to play with different tables/software considering you are in the water at this point. Solutions #1 get a good dive computer and use tables as a back up. Since this wasn't an option,then solution #2 maybe go from a deep stop profile to a on the fly Buhlman 16 by compressing more time into those shallower stops. Bruce Weinke's research has shown that either the deep stop or shallow stop profiles are good,just don't try to modify them ie adding deep stops to a Buhlmans. I would add a lot of time to the 20' on oxygen since that is the better inert offgassing depth,and maybe eliminate the 10' stop in favor of the 20'.
Hypothetically:
Stop at 70ft 3:00 (26) Trimix 18/45 0.56 ppO2, 15ft ead, 24ft end
Stop at 60ft 4:00 (29) Trimix 18/45 0.51 ppO2, 11ft ead, 18ft end
Stop at 50ft 5:00 (33) Trimix 18/45 0.45 ppO2, 6ft ead, 13ft end
Stop at 40ft 7:00 (39) Trimix 18/45 0.40 ppO2, 1ft ead, 7ft end
Stop at 30ft 9:00 (47) Trimix 18/45 0.34 ppO2, 0ft ead, 2ft end
Stop at 20ft 22:00 (53) Oxygen 1.60 ppO2, 0ft ead
Stop at 10ft very slow ascent from 20' to surface on oxygen
This is probably very conservative,but sticking with the true nature of the problem,I planned it on the fly with no reference.

 

[rjack321]

Sticking with the OP's issues
Blowing deep stops with a partial rapid ascent is not something I would try to fix "later" in the profile.

Reason being is that by the time I get to 20ft I will either have a bubble (from the rapid ascent/missed stop) that has grown substantially or I won't. If I did form that bubble deep I probably already have type 2 DCS on its way. (Fast tissues, rapid ascent at depth being much more likley to affect CNS tissues than a joint).

Sooo, my plan would be to stay at 100ft until my runtime has caught up to where I should be at 100ft (e.g. 4 extra mins, until 21min RT). Its suboptimal but minimizes the risk from the rapid ascent and missed stops alot more than continuing on the shortened schedule and adding time late in the profile. I would add a few mins of O2 time as a buffer like you did (probably 5mins extra on O2), but you really need to mitigate the missed stops deeper not wait until you're shallow.

To me, this schedule 4min error is actually quite minor. The 60fpm ascent on 18/45 is much more concerning.

 

[Rick Murchison]

... The 60fpm ascent on 18/45 is much more concerning...

Well, it did save a couple hundred bucks
Rick

 

[boulderjohn]

To me, this schedule 4min error is actually quite minor. The 60fpm ascent on 18/45 is much more concerning.

I do not wish to come across as more expert on this than I am, so what follows is more of a question and request for more information than a challenge.

The is a rather quick jump from 7.5 ATA to 4 ATA, but by a lot of schedules people have used it would be very acceptable. My reading indicates that some people think that in the first part of an ascent, 60fpm, while certainly not preferred, is not that dangerous. Remember that Haldane's early theories indicated that going to half the ATA pressure like this could be done quickly. That theory was later adjusted, but not by a lot.

And I do understand that it is the existing bubbles, not the dissolved gas we are concerned about.

 

[rjack321]

I do not wish to come across as more expert on this than I am, so what follows is more of a question and request for more information than a challenge.

The is a rather quick jump from 7.5 ATA to 4 ATA, but by a lot of schedules people have used it would be very acceptable. My reading indicates that some people think that in the first part of an ascent, 60fpm, while certainly not preferred, is not that dangerous. Remember that Haldane's early theories indicated that going to half the ATA pressure like this could be done quickly. That theory was later adjusted, but not by a lot.

And I do understand that it is the existing bubbles, not the dissolved gas we are concerned about.

Haldane really didn't address trimix. Buhlmann tried to and still didn't get it right so the GFs got added pushing the time even deeper.

I think of deep stops like insurance. You can get away without them for years. But when the time comes you really need them. Missing deep stops or fast ascents from depth are not going to result in "minor" fixable hit to a joint.

Links to one well publicized incident of rapid ascent / missed deep stops and the resulting type 2 hits experienced before they even got to 20ft.
The Deco Stop

Granted the deviations shown are much bigger than 3-4mins and 100 vs. 130ft first stop depth. But in this case there's little to be lost be extending the 100ft time to at least the proposed runtime and much to be avoided.

 

[rjack321]

Well, it did save a couple hundred bucks
Rick

I don't get it... How?

 

[Thalassamania]

I'm with Rjack, I'd stay put and let my runtime catch up with me.

 

[boulderjohn]

Haldane really didn't address trimix. Buhlmann tried to and still didn't get it right so the GFs got added pushing the time even deeper.

I think of deep stops like insurance. You can get away without them for years. But when the time comes you really need them. Missing deep stops or fast ascents from depth are not going to result in "minor" fixable hit to a joint.

Links to one well publicized incident of rapid ascent / missed deep stops and the resulting type 2 hits experienced before they even got to 20ft.
The Deco Stop

Granted the deviations shown are much bigger than 3-4mins and 100 vs. 130ft first stop depth. But in this case there's little to be lost be extending the 100ft time to at least the proposed runtime and much to be avoided.

I guess I am still interested in the big question: why?

Interestingly enough, the Deco Stop thread you cite, which I read when it first came out, was in the back of my mind when I wrote the post. If you read the whole thread rather than that one part of that one post, you see that if there is any kind of a consensus, it is that more rapid ascents are acceptable on the deeper portion of the dive, and the percentage of change in ATAs is important to consider as well. I agree that they are talking about the deeper portion of the dive being a little deeper than this example, but not to the degree that it is out of the question.

 

[RJP]

I'm with Rjack, I'd stay put and let my runtime catch up with me.

Pretty new to this sort of diving, but gun to my head (needing to "re-plan" at depth) I think that's what my approach would be as well, extending 20ft stop on O2 as well.